In electronic circuitry, an optimal drive circuit will provide voltage in excess of the breakdown voltages of the individual circuit components. For example, the cascode circuit, a cascaded transistor arrangement provides an output voltage which is up to two times (2.times.) the value of the breakdown voltages of each of the circuit's transistors. Such an output is particularly advantageous in an Electrically Erasable Programmable Read Only Memory (EEPROM) semiconductor circuit for performing a write operation, for example, by supplying a large voltage to the EEPROM in an efficient manner.
A simplified version of a cascode transistor circuit for providing a large output voltage can be seen in FIG. 1. In the FIG. 1 circuit, an input voltage of 5V from location 10 is provided to the gate of a first transistor 14. The gate of transistor 12 is connected to ground potential and the drain is connected to the source of the first transistor 14. For the sake of the example, resistor 16 is provided as a 10V load for the circuit, although in practice a p-channel device would generally be provided as the load. The output at node 18 will be from 0V to 10V, while each individual transistor, 12 and 14 each of which has a breakdown voltage of 5V, will see only 5V. Therefore, the cascade transistor arrangement is capable of providing a drive voltage of approximately 2.times. the breakdown voltage capability of the individual transistors.
The output voltage in semiconductor transistor circuits can be enhanced by increasing the breakdown voltages of the individual transistors, which can be accomplished by modifying the processing steps when fabricating the circuit components. For semiconductor transistors, using thicker gate oxide and tailoring the junction during ion implantation can improve the breakdown voltage of the individual transistors. A disadvantage to the known process modifications for enhancing the output voltage of drive circuits is that each process change is expensive in terms of monetary cost, time expenditure, and compromise to other aspects of circuit performance.
Implementation of cascode transistor circuits in silicon-on-insulator (SOI) substrates is desirable due to the dielectric isolation between devices. However, optimal performance of SOI cascade transistor structures is not realized because the breakdown voltages are reduced by the floating body effect encountered in SOI.
While transistors are detailed herein, it will be clear to one having skill in the relevant art that the foregoing, and ensuing, descriptions can be applied to any three terminal amplifier, wherein a certain input voltage is applied to one terminal and the output current is controlled at the two output terminals, such that the device has a breakdown voltage which cannot be exceeded. While the three terminal amplifiers most commonly used today are transistors, the invention can be applied to other three terminal amplifiers, such as vacuum tubes.
It is therefore desirable, and is an objective of the present invention, to devise an improved drive circuit which will provide enhanced operating voltage.
It is another objective of the present invention to produce an improved semiconductor drive circuit having enhanced output voltage without increasing the cost of semiconductor processing.
It is yet another objective of the present invention to provide an improved semiconductor drive circuit which can readily be implemented under existing silicon-on-insulator (SOI) C-MOS technology.